Increase Health Vial (Glitch)
When approaching a checkpoint do not mark it, jump over it and search the area for any Vampire Relics or any Lore you can get from fights. Then go back and activate the checkpoint. Save the game then reload. All the relics will be restored and all the peasants/guards will be alive again.

Note: Once your Health Vial is the same as your Lore Vial do not exceed the amount in your Lore bar because your Health will reset to what it was at the start of the game, making it impossible to finish the game.

Start with Soul Reaver and Iron Armour
Enter this at the main menu:
L1, R1, L2, R2, square, circle, triangle

Then start a new game and Kain will have the Iron Armour from Blood Omen and the Soul Reaver as well.

Note: You cannot pick up any other weapons, and the Soul Reaver is indestructable.

All Dark Magic
Press Pause and press and hold L2. Now press square, triangle, circle, and X (2x). Play further by unpausing.

Extra Movie
Go to the main menu and type in up (2x), down, right and left